Ceramic dielectric with high dielectric constant and process of making the same



United States Patent CERAMIC DIELECTRIC WITH HIGH DIELECTRIC CONSTANT AND PROCESS MAKING THE 'SAlVIE No Drawing. Application June 17, 1953,

Serial No. 362,412 v i Claims priority, application France July 1, 1952 3 Claims. 01. 106-39) The present invention relates to ceramic dielectrics of high dielectric constant, more particularly to such ceramics as are characterized by crystalline lattice of the perovskite type. I I

If certain difliculties now to be pointed out could be overcome, ceramic materials of the foregoing type that have a dielectric constant of several thousandwould be particularly worthwhile for the manufacture of electric capacitors. v

A ceramic block that has first been baked is much too hard to lend itself readily to subdivision into pieces of the shape and accurate dimensions required for capacitor and other precision dielectrics. On the other hand, dielectric pieces for such use that have been shaped from a paste to predetermined form (such as tubes or thin plates), give rise to technological problems in assuring maintenance after baking of the requisite precision of di- 1 mensions and shape. For in the course of baking, these pieces suffer contraction which at times is very substantial and depends" on factors unknown to the ceramic manufacturer; such for example as the origin of these components, their physico-chemical state and the heat treatment to which the basic components had been submitted, which may vary from one batch to another, etc.

In the manufacture of tubes and particularly of thin plates, it is important for technological reasons to use pastes that have a very low contraction, a requirement not usually compatible with the desired imperviousness of the baked product, for the greater the contraction the greater the imperviousness.

Such contraction and other'objections are particularly or for low power factor in any required frequency range, all depending upon the specifications required for capacitor dielectrics or similar use and with close tolerances, all at relatively low cost without the need for expensive equipment or highly skilled supervisory talent and indeed with high output by resort to simple and more or less conventional oven equipment.

According to the invention, there are produced ceramic pieces that suffer only negligible contraction in the course of the baking and are well vitrified, all by controlling the choice of components and their proportions under definite thermal treatments.

According to the invention, a mixture of barium titanate and strontium titanate proportioned by weight as follows: i

, Parts BaTiOs 70 to 98 SrTiOs 30 to 2 is baked to a temperature between l200 and 1400 C.

The resultant product is crushed to a powder, to which is added 1 to 10 parts of powdered cerium oxide per 100 parts of the mix of which a paste is made by adding an organic binder which may have both a thermoplastic and a. thermosetting component. 'As taught in French Patent No. 1,050,129 above referred to, a suitable binder of this type is, for example, polyvinyl chloride and tricresyl phosphate as thermoplastic materials and a phenolformaldehyde resin as a thermosetting material. The mixture with the oxides may befacilitated by the addition of cyclohexanone. Other suitable organic binders may comprise (as thermoplastics) stearin, cellulose derivatives, polyamides, polystyrene, polyvinylidene, polyviny'lacetate and butyrate, methacrylic resins and silicones; and (as thermosetting materials) phenol plastics, amino plastics, polyesters and certain silicones. The paste is subdivided into pieces of desired form and dimensions and these pieces are then baked at temperature between 1200 and 1400 C.

The paste, regardless of the binder used, is sheeted while hotand little plates such as used for capacitor dielectric for instance, are cut from the sheets thus obtained and such little plates are then introduced into the baking oven.

The resultant product composed of barium titanate,

., st1-ontium titanate and cerium oxide in the proportions disturbing with the procedure described in the applicaissued as French Patent No. 1,050,129.

In that procedure there is mixed with the mineral above defined, is well vitrified and quite impervious.

The composition of barium titanate and strontium titanate may be produced from a mixture of powders of powders an organic binder which has both a thermo- Y plastic and a thermosetting component. The paste thus obtained is brought into sheet form by use of a laminator, the rollers of which are heated to a temperature at which the binders soften. The sheets are cut into little plates which are baked in an oven, the temperature of or higher as required in many capacitor uses, and the power factor may not be as low as desired for many capacitor uses.

It is among the objects of the invention to provide ceramic dielectric of high dielectric constant, the performance of which can readily be controlled formaximum dielectric constant at ambient temperature or higher per ature or higher,'the components are used in the following proportions by weight:

Parts BaTiOs 92 to 97 SrTiOs 8 to 3 When a product is desired of lowest possible power factor (1 percent or even less), the components are used in the following proportions by Weight:

Parts BaTiOs to SI'TiOs 25 to 15 Where the quality of the product is subject to closest tolerance with the formulations stated in the last two paragraphs, there is also used 3 to 5 parts of cerium oxide for 100 parts by weight of BaTiOs and SrTiOs.

To make a product, the composition of which is above r. r u L :3 defined, there may be used a mixture of titanic oxide (TiOli), b'a'riii'r'ri carbonate and s't'rzshnnai "c'arb'ciii'a'f iii the suitable proportions above referred to. This mixture is baked at highjernperature (between 1290f. ,Q and tauofi cq, lld the r silient productfis ,tlien brbken; crushed and screened (screen of 100 mesh per square millimeter or equivalent). 1 e r M V v To that powde'r, whether prepared titahates q'r y the procedure described inth e pfr'evioiis pa agraph, there is then added a quantityof ceriuin oxide predeterminedrin manner above suggested to pjrolvide a compositjon of definitive formula Theniixtu're i and as a paste with water in ball mills, both bowl and balls being po li A t 7 r r f t The paste thus obtained is then dried; screened and mixed with anorg a nic binder which permits torming by extr sion, by, molding 9; if thin plates are to be made, by sh'e'etlng. These pieces are then baked at temperatures between 1200 and 1400 C.

By way of non-limitative examples, the two following specific compositions by weight are desirable:

Formula r p BaTiO; 95 SrTiO: C602 2 This mixture prepared from very pure products, is baked at 1250 C.

Formula 2 BaTibs 82 SI'TiOs 18 COz .L.L. 4

shape-d, formed and then baked, preferably at about 1 1320 c. v p H p To co mplete the capacitor from the ceramic dielectric described, two opposed surfaces of said dielectric are silvered by means of a brush with a paint of silver base. This coating is then baked at about 750 C. so that the metallic silver shall adhere firmly to the ceramic to form itselectrodes without the intervention of plastic material, For Formula 1, the dielectric constant is about 4000 4 C. The power factor is very low at 20 0., about 012 perc'efit at 'o'ii kil'ocy't'rl'e and even lower, about 0.1 percent at one megacycle. L

As many changes could be made in the above product and process, and niaiiy apparently widely ditferent embodiments of .this invention could be made without departing frolrn the scope of the claims it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Having thus described rour invention, what we claini as new and desire to secure by Letters Patent (at the United States is:

1. Process of manufacturing a ceramic, dielectric prod uct by baking betweenl200" and 1400" C. a mixture corresponding to the following components by weight:

- Parts BaTiOs 70 to 98 SI'TlO3 i I 30 to 2 and 1'4'O0 'C. a mixture corresponding to the following components by weight:

r 1 Parts BaTiO3 92 to 97 SrTiOw 8 to 3 crushing the rejsultant product to a powder, adding thereto 1 to 10 p art s by weight of'CeOz for 100 parts of said product, for rnin g a paste fro m said resultant mix with anorganic binder, shaping the paste into an article of desired form and baking that article at a temperature at ZQf c. with a maximum Of 6500 at c. The dielectric losses are of the order of 0.5 percent at one kilogram whether pure components have been used, because these are characterized by a square lattice while with the use of commercial products of ordinary quality, the lattice is cubic 1500 at 20 C. It reaches a maximum toward minus Fqt hil q cons rain 'spti ja am between l 200f 14001 C, in oxidizing atmosphere.

fi Proc ess oi manufacturing a ceramic dielectric product, characterized by aminimurn power factor, by baking between 1 2 00 and 1400' C, a mixture corresponding to the following components by Weight:

1' n 7 Parts B aTi O3 to SrTiOs 25 to 15 Reie'ren'ce's Cited in the tile of this patent UNITED STATES PATENTS than; Waia j ne 15,194 2520 376 Bonnet 1. Aug- 29, 1950 2,616,813 Kla sens Nov. 4, 1952 

1. PROCESS OF MANUFACTURING A CERAMIC DIELECTRIC PRODUCT BY BAKING BETWEEN 1200* AND 1400*C. A MIXTURE CORRESPONDING TO THE FOLLOWING COMPONENTS BY WEIGHT: 